Shoulder holster would be my first choice. There are a ton of rigs with mag pouches held under the strong arm.
Or, perhaps one of the Chest/crossdraw hoslters that are popular with pilots.
Galco used to make a "driving" holster that might be comfortable, but I don't know how scure it is for mounting/dismounting your steed.
Of course, you could contact Scott Evans and have him make you a custom horse rig.
[This message has been edited by Rob (edited 02-11-99).]